The present invention relates to a screening mask for screening patterns on substrates, and more particularly, relates to a screening mask having stress-relieving areas to prolong the life of the screening mask.
In the fabrication of packages for semiconductor devices and the like, it is necessary to deposit finely detailed wiring patterns of conductive material on insulative substrates such as those made from ceramic materials. The conductive patterns typically consist of wiring lines, via, pads and the like. In order to provide these conductive patterns, conductive paste is deposited on the insulative substrate through a screening mask.
Screening masks are typically made of a metallic material such as molybdenum, copper, nickel and the like. However, the material of the screening mask is not important to the present invention.
While the specific screening mask material is not germane to the present invention, it must nevertheless be sufficiently robust to withstand the rigors of screening. Screening masks are very thin, typically on the order of 2.5 to 4 mils, and are held in a frame during screening. The screening mask is drawn tight across the frame such that the screening mask is taut and held in tension. The conductive paste that is used for screening is typically very viscous. In one form of screening, so-called extrusion screening, the very viscous conductive paste is extruded through the apertures of the screening mask.
Separating the apertures of the screening masks are tabs which provide structural rigidity to the screening mask. The process of extrusion screening puts tremendous stresses on the screening mask, often causing the tabs to break. When a tab breaks, the screening mask must be discarded. Tab breakage is worst with screening masks having a high density wiring pattern. The pass factor, i.e., the number of substrates screened without breaking a tab, consequently decreases with an increase in wiring pattern density.
A conventional screening mask is disclosed in Comino et al. U.S. Pat. No. 6,095,041, the disclosure of which is incorporated by reference herein. As can be seen in FIG. 1 of the reference, the screening mask is principally open for screening of features except for tabs bridging the open features so as to provide structural rigidity to the screening mask.
Randall U.S. Pat. No. 5,763,121, the disclosure of which is incorporated by reference herein, discloses a lithographic stencil mask having a plurality of non-uniformly distributed stress relief openings in the active area of the stencil mask which relieve and equalize tensile stresses within the mask, thereby compensating for any non-uniform distribution of the pattern openings in the stencil mask.
Storm U.S. Pat. No. 6,103,427, the disclosure of which is incorporated by reference herein, discloses a pellicle having a continuous, tortuous path between the interior and exterior of the frame and gasket assembly so as to enable pressure equalization between the two spaces.
Notwithstanding these advances in the art, there remains a need for a screening mask design which substantially alleviates the problem of tab breakage.
Accordingly, it is a purpose of the present invention to have a screening mask design which substantially alleviates the problem of tab breakage.
It is another purpose of the present invention to have a screening mask design which has an increased pass factor.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.
The purposes and advantages of the present invention have been achieved by providing, according to a first aspect of the present invention, a screening mask having a stress-relieving area comprising:
an inner functional area having a pattern which is replicated on an underlying substrate, the inner functional area pattern having at least one open area through which a paste is extruded and at least one tab; and
an outer nonfunctional area distinct from the inner functional area, the outer nonfunctional area having at least one stress-relieving area adjacent to an edge of the screening mask that protects the at least one tab in the inner functional area from breaking.
According to a second aspect of the present invention, there is provided a screening mask assembly comprising:
a paste dispensing apparatus;
a screening mask frame; and
a screening mask mounted in the frame, the screening mask having an inner functional area having a pattern which is replicated on an underlying substrate, the inner functional area pattern having at least one open area through which a paste is extruded and at least one tab, and the screening mask having an outer nonfunctional area distinct from the inner functional area, the outer nonfunctional area having at least one stress-relieving area adjacent to an edge of the screening mask;
wherein, in operation, when a paste is dispensed over and into the screening mask by the paste dispensing apparatus, stresses in the screening mask are absorbed by the at least one stress-relieving area, thereby preventing breakage of the at least one tab of the screening mask.